Conventional golf balls can be divided into two general types of groups: solid balls or wound balls (also known as three piece balls). The difference in play characteristics resulting from these different types of construction can be quite significant. Balls having a solid construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. Solid balls are generally made with a single solid core usually made of a cross linked rubber, which is enclosed by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents and is covered by a tough, cut-proof blended cover. The cover is generally a material such as SURLYN.RTM., which is a trademark for an ionomer resin produced by DuPont. The combination of the core and cover materials provide a "hard" ball that is virtually indestructible by golfers. Further, such a combination imparts a high initial velocity to the ball which results in increased distance. Because these materials are very rigid, solid balls can have a hard "feel" when struck with a club. Likewise, due to their construction, these balls have a relatively low spin rate which provides greater distance.
At the present time, the wound ball remains the preferred ball of the more advanced player due to its spin and feel characteristics. Wound balls typically have either a spherical solid rubber or liquid center core around which many yards of a stretched elastic thread are wound. The wound core is then covered with a durable cover material such as a SURLYN.RTM. or similar material or a softer cover such as Balata or polyurethane. Wound balls are generally softer and provide more spin, which enables a skilled golfer to have more control over the ball's flight and position. Particularly, with approach shots onto the green, the high spin rate of soft, wound balls enable the golfer to stop the ball very near its landing position.
Regardless of the form of the ball, players generally seek a golf ball that maximizes total game performance for their requirements. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with a wide variety of performance characteristics to meet the players individual requirements. Thus, golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance for golfers of all skill levels.
To meet the needs of golfers with various levels of skill, golf ball manufacturers are also concerned with varying the level of the compression of the ball, which is a relative measurement of the golf ball stiffness under a fixed load. A ball with a high compression feels harder than a ball of lower compression. Wound golf balls generally have a lower compression which is preferred by better players. Whether wound or solid, golf balls typically become more resilient (i.e., have higher initial velocities) as compression increases. Manufacturers of both wound and solid construction golf balls must balance the requirement of higher initial velocity from higher compression with the desire for a softer feel from lower compression.
To make wound golf balls, manufacturers use winding machines to stretch the threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. Generally, as the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more resilient wound ball is produced, which is desirable.
Referring to FIG. 1, a conventional golf ball thread 10 is shown. In general, a single-ply golf ball thread or two-ply thread 10 is formed and wound around a center. Single-ply threads are generally made using a liquid latex that is cast into a sheet and then slit into threads having a generally rectangular or square cross-section. Two-ply threads are generally made by mixing synthetic cis-polyisoprene rubbers, natural rubber and a curing system together, calendering this mixture into two sheets, calendering the sheets together, curing the sheets to vulcanize and bond the sheets together, and slitting the resultant sheet into threads having a generally rectangular or square cross-section. Another method of forming threads is an extrusion method. However, extruded thread has not been used in golf ball applications. An example of an extruded thread that is not used in golf balls is disclosed in U.S. Pat. No. 5,679,196 issued to Wilhelm et al. This patent discloses a thread formed of a mixture that has more than 50% natural rubber.
A number of different windings have been disclosed for use in golf balls. U.S. Pat. No. 4,473,229 to Kloppenburg et al discloses a golf ball having a core formed of graphite fibers and windings made of graphite filaments and resins. Yarns are made with the graphite filaments and resins, and as many as four or more yarns are combined to form a final yarn used for winding. U.S. Pat. No. 5,713,801 to Aoyama discloses use of a layer of high tensile elastic modulus fibers wound about the core. The fibers have a tensile elastic modulus of at least 10,000 ksi. Also, U.S. Pat. No. 5,816,939 to Hamada et al. discloses a rubber thread for winding with a tensile strength retention of .gtoreq.70%, a hysteresis loss of .ltoreq.50%, and an elongation of 900-1400%.
Prior art wound golf balls and cores typically use polyisoprene rubber thread. The polyisoprene thread is wound onto the cores at elongations between 500-1000%. The amount of thread required for a golf ball core is dependent on the elastic modulus of the thread in the elongated state. Elongated polyisoprene thread has an elastic modulus between 10 and20 ksi. Further, the properties, in particular resilience, of the wound ball or core are dependent on how well the thread packs during winding. The dimensions of the thread control the packing density. Present art polyisoprene threads are typically 1/16" wide by 0.02" thick, measured prior to winding. However, present are polyisoprene thread is commonly produced in thickness between 0.014" and 0.024".
There are some drawbacks to the conventional single-ply threads used in golf balls. The single-ply occasionally contains weak points. As a result, manufacturers of wound balls do not wind using the maximum tension or stretch the thread to the maximum elongation, because to do so would cause an excessive amount of breakage during winding. When a thread breaks during manufacturing, an operator must restart the operation. This decreases production, and is thus undesirable. The use of two-ply threads in golf balls reduces but does not eliminate this problem.
The thread can also break during play due to impact of a club with the ball. These breaks can result in various consequences. The cover material is disposed between the thread portions adjacent the cover. When the thread portions adjacent the cover break, the cover material tends to hold these thread portions in the proper position. However, if enough thread portions break near the cover, a lump will be created on the outside surface of the ball, which makes the ball unplayable.
More severe problems can occur, when the thread portions near the center break. In a wound ball with a solid rubber center, the resilient rubber of the center is relatively soft compared to the hardness of the highly stretched thread portions. After a thread portion adjacent the center breaks, the thread portion can contract and cause a loss of compression and resiliency. This results in a distance loss, which is undesirable.
In a wound ball with a fluid-filled center, after a thread portion adjacent the center breaks, the resultant imbalance in stress adjacent the center causes the thread to cut through the envelope that contains the fluid. This destroys the structural integrity of the ball and makes it unplayable. If this type of failure happens during a shot, it can result in a short shot. It can also result in the ball deviating from its line of flight as it leaves the club, so that the ball can end up off the fairway. Both of these consequences are undesirable.
Therefore, golf ball manufacturers are continually searching for new ways in which to provide wound golf balls that deliver the maximum performance for golfers while decreasing the occurrence of thread breaks both during manufacturing and during play. It would be advantageous to provide a wound golf ball with a lower compression, higher initial velocity, more dense packing, improved durability, and improved manufacturing processibility. The present invention provides such a wound golf ball.